Videos are much better when you add music. For example, suppose you want to add a song you have on your device. Place the red line at the bottom of the page where the song should begin. Tap the music icon and select the song.
When you select a song, a red plus sign appears. The sound plays so you can confirm that it is the song you want to add. When you are sure that it is the song, press the red plus sign and the name of the song will appear below.